The invention relates to a hybrid insulation material composed of aerogel and rigid ceramic fiber materials and methods for their production, including ceramic fiber insulating materials impregnated with aerogel and other nanoporous materials. More specifically, the invention relates to low-conductivity ceramic fiber insulating materials for use on reusable launch vehicles.
Reusable launch vehicles (RLVs), such as the space shuttle, repeatedly travel into or beyond the Earth""s upper atmosphere and then return to the Earth""s surface. During flight, the RLVs experience extreme temperatures, ranging from xe2x88x92250xc2x0 F. while in orbit to over 3000xc2x0 F. upon reentry to the atmosphere. Because of the extreme temperatures, the vehicle and its contents must be protected by a thermal protection system. The thermal protection system is an outer covering of insulation, the purpose of which is to prevent the body of the vehicle from reaching a certain maximum temperature. For the space shuttle, the maximum temperature is about 450xc2x0 F., the temperature at which the aluminum structure of the shuttle begins to weaken.
Thermal protection systems for RLVs are constructed from a large number, usually several thousand, of insulative rigid tiles and blankets. The tiles, which are used mostly on the lower surface due to its smoother surface, function to insulate the vehicle from the environment and to radiate and reflect heat from the vehicle. In addition to protecting the vehicle from environmental heat sources, the insulative tiles also provide protection from localized heating from such sources such as the vehicle""s main engine, rocket boosters and directional thrusters.
RLVs such as the space shuttle typically utilize a variety of tiles to cover the lower surface of the vehicle. Different areas of the vehicle encounter different heat profiles and different physical stresses during flight. Therefore, a variety of tiles having different compositions, densities, and coatings are placed at different positions of the vehicle depending on if such positions are leeward or windward, upper or lower surfaces, etc. The most predominate tiles used today on lower surface are Lockheed Insulation (LI) and Alumina Enhanced Thermal Barrier (AETB) are used on the base heat shield due to its relatively higher thermal conductivity.
The Lockheed Insulation materials are comprised of high purity amorphous silica fiber. To produce the Lockheed Insulation, a slurry of the silica fibers having a diameter of 1 to 3 xcexcm is formed in deionized water with a V-blender. The slurry is mixed with ammonia and stabilized colloidal silica solution after which it is placed in a casting tower where it is dewatered and slightly pressed to remove a portion of the water. The partially dried slurry is heated to a temperature of 250xc2x0 F. to remove the remaining residual water. The dried silica composition is then fired to a temperature of up to 2300xc2x0 F., which causes the colloidal silica to sinter the silica fibers to one another. The resulting insulative material is a low density mass of randomly arranged fused silica fibers. By selectively pressing the silica fiber slurry and subjecting to different firing temperatures, various densities of the resulting dry silica material may be produced. The Lockheed Insulation tiles are marketed under the trade names LI-900(trademark), LI-1500(trademark) and LI-2200(trademark), having densities of 9 lb/ft3, 15 lb/ft3 and 22 lb/ft3, respectively.
The Alumina Enhanced Thermal Barrier (AETB) consists of about 68 percent silica fiber, about 12 percent Nextel fiber (a combination of alumina, silica, and borate), about 20 percent alumina fiber, and about 2 percent silicon carbide. The fiber diameter ranges from 1 to 3 xcexcm for silica and alumina fibers, and from 5 to 10 micron for Nextel fibers. The processing is very similar to the Lockheed Insulation. Colloidal silica is not added to the AETB material before firing. Instead, high temperatures experienced during firing cause the borate within the Nextel fiber to form boron oxide, which fuses to the fibers and sinters the ceramic fibers to one another. The AETB material is commonly marketed in the forms of AETB-8(trademark), AETB-12(trademark), AETB-16(trademark), and AETB-20(trademark) tiles, having densities of 8 lb/ft3, 12 lb/ft3, 16 lb/ft3 and 20 lb/ft3 respectively.
Because of its extraordinary low thermal conductivity, LI-900(trademark) insulation tiles are used on the lower surface of most RLVS. The pure silica fiber skeleton of LI-900(trademark) tiles is capable of remaining in tact up to temperatures of 2500xc2x0 F., which exceeds the maximum temperature (2300xc2x0 F.) experienced by RLVs during reentry into the Earth""s atmosphere. LI-900(trademark) insulation, however, suffers from two main disadvantages. First, it suffers from severe shrinkage after exposure to temperatures above 2500xc2x0 F. and for long periods of time. Shrinkage along the mold line of the RLV leads to widening gaps between the insulating tiles as well as surface recession and thus increases heating at the inner mold line. Second, LI-900(trademark) and other Lockheed Insulations are not compatible with the tough coating, TUFI (toughened unipiece fibrous insulation) which is needed for improved surface durability. Application of TUFI coating results in slumping of the pure silica insulation. Because of incompatibility with the tough coating, LI-900(trademark) materials are easily susceptible to damages during flight or servicing of the RLV.
Unlike LI-900(trademark) insulation, the AETB material is compatible with the TUFI coating. As a result, the AETB is a much more durable tile which requires less frequent replacement. AETB, however, is more thermally conductive than the Lockheed Insulation materials. As a result of the increased thermal conductivity, the AETB material is unable to protect the RLV substructure from temperatures experienced during reentry. Therefore, AETB may not be used on much of the lower surface of the RLVs.
What is needed is a ceramic fiber insulative material having the same or lower thermal conductivity found in LI-900(trademark) insulation while exhibiting the durability, strength and dimensional stability of AETB tile material.
The present invention is an insulating material for use in extreme temperatures having a variety of applications, but designed for the protection of reusable launch vehicles (RLVs). The insulating material is a unique combination of a substrate of sintered ceramic fibers which form a low density, highly porous material and an aerogel or other nanoporous material which impregnates at least a portion of the porous ceramic substrate. The resulting insulation has very low thermal conductivity (lower than a LI900 tile). Additionally, the insulation exhibits sufficient tensile strength, good dimensional stability, and good compatibility with the TUFI coating to withstand damage typically suffered during flight and servicing of the RLV.
The basis of the invention is the combination of a porous ceramic tile substrate with a low density nanoporous material such as silica- or alumina-based aerogel. The porous tile substrate of one embodiment includes 60 to 80 wt % silica (SiO2) fibers, 20 to 40 wt % alumina (Al2O3) fibers, and with 0.1 to 1.0 wt % boron-containing constituent as the sintering powders. The silica-based or alumina-based nanoporous material typically has a density of about 1.0 lb/ft3 to about 10 lb/ft3.
The boron-containing constituent contained in the tile substrate provides boron-containing by-products which act to fuse and sinter the silica and alumina fibers of the substrate when heated. No supplemental binder is required during production of the insulative material
A preferred embodiment of the tile substrate composition is 65 wt % to 75 wt % silica fibers, 25 wt % to 35 wt % alumina fibers, and 0.1 wt % to 0.5 wt % boron-containing constituent. A particularly preferred tile substrate composition is 67 wt % silica fibers, 32.75 wt % alumina fibers, and 0.25 wt % boron-containing powders such as boron carbide (B4C).
The tile substrate material is produced by first dispersing the ceramic fibers and then a boron-containing constituent in an aqueous slurry. The slurry is blended with a shear mixer which chops and disperses the fibers evenly throughout the slurry.
Prior to formation of the insulative substrate, the fiber slurry is optionally processed through a separation means in order to remove undesirable solids, known as inclusions or shots from the fiber slurry suspension. The insulative properties of the material stems from having small diameter ceramic fibers surrounded by large volumes of air. High density ceramic shots or clumps of fiber are detrimental to the effectiveness of the insulation, and are therefore removed before casting.
After separation of inclusions, if applicable, the slurry is drained and pressed. Drainage is accomplished by transferring the slurry to a casting box where excess water from the slurry is allowed to drain from the casting box through the porous bottom of the container. Drainage of the water may be accelerated by applying a vacuum to the bottom of the casting box. The slurry is pressed to produce a wet billet of ceramic fiber. The slurry is preferably pressed vertically, by moving a top surface downwards upon the fibers, pressing them against a bottom surface.
After pressing, the wet billet is dried and fired. The drying step removes residual water from the billet. The firing step fuses the fibers to one another. Drying occurs at approximately 200xc2x0 F. to 500xc2x0 F. for 24 to 36 hours. Firing occurs at a temperature between about 2300xc2x0 F. and about 2600xc2x0 F. for 1 to 5 hours.
After firing, a tile substrate is machined to final size and then either is subjected to coating process or to a nanoporous material impregnating process. The aerogel is a nanoporous substance which resides between the fibers of the substrate and prevents the conduction or radiation of heat through the insulation in the spaces between the fibers. Thus, the nanoporosity of the aerogel utilizes the insulative capacity of the air trapped within its pores while substantially limiting the ability of air to conduct heat through the pores.
The aerogel is prepared in three steps. The first step is preparation of solution via hydrolysis of silicon alkoxide compounds for silica aerogel or aluminum alkoxides for alumina aerogel in a compatible solvent. The second step is a base or acid catalyzed gellation of the solution and impregnation of the solution into the tile substrate prior to gel formation such that the gel forms within the tile substrate. The last step is solvent removal to produce the dried aerogel within the tile substrate.
The drying step may use supercritical extraction to remove the solvent by applying pressure and temperature (depending upon the type of solvent) or other solvent exchange or evaporative drying methods. With supercritical extraction, the typical processing cycle is about 4 to 9 hours. In a preferred embodiment, a silica gel solution is formed from TMOS (tetra-methoxy silane), methanol, water, and a base catalyst. In another embodiment, the alumina gel solution is formed from aluminum tri-sec-butoxide, ethanol, water, and an acid catalyst (for example see, B. Himmel, Th. Gerber, H. Bxc3xcrger, G. Holzhxc3xcter, A. Olbertz, J. Non-Cryst. Solids 186 (1995) 149-158). The sol solution is injected into the entire substrate or a portion thereof, and the impregnated substrate is placed into an autoclave for the supercritical drying step either prior to or after gellation of the silica or alumina solution in the tile substrate. Typically, during the drying step, the temperature and pressure are raised to about 625xc2x0 F. and about 2000 psi over about a 1-3 hour period. After thermal equilibrium is reached and about a 1 to 8 hour decompression period, the dried aerogel is set within the substrate, forming the invented insulative material.
Alumina aerogels, which can withstand higher temperatures than the silica aerogels, may be impregnated throughout the thickness of the substrate. Its higher temperature stability allows it to be able to withstand the coating firing temperature of RCG/TUFI without slumping or cracking. Thus, the aerogel impregnation can takes place before the coating process. For high temperature application above 2000xc2x0 F., the silica aerogel, on the other hand, is preferable to impregnate only about halfway from the inner mold line portion of the material. Partial thickness treatment prevents the silica aerogel from encountering the most extreme temperature conditions ( greater than 2000xc2x0 F. and upward), under which it could slump and shrink within the insulation material. To minimize shrinkage, the tile substrate is first coated with the RCG/TUFI combination prior to impregnating with the silica aerogel.
The production of tile/aerogel insulation typically follows the following sequence. For alumina aerogel impregnated tile, for example, the porous tile is first cast and fired to yield billets having a density of approximately 6.0 to 20 lb/ft3. Final tile configuration is then machined from the billet. The tile is machined so that the inner surface and the outer surface of the tile are roughly parallel to the in-plane direction of the aligned fibers. This arrangement provides the lowest thermal conductivity between the outer and inner mold lines. The final machined tile is then fully impregnated with alumina aerogel. Then it is subjected to RCG/TUFI spraying and firing processes. After firing, the tile product has bulk density ranging from 8 to 25 lb/ft3.
The insulative material has shown very low thermal conductivity, particularly in the through-the-thickness direction. The primary reason is due to the impregnated nanoporous material occupying small pores between the fiber-to-fiber, effectively blocking convective and radiative heat transfer.